ENHANCEMENT OF THE FORCE MEASUREMENT CAPABILITY AT IPT MODERNIZATION OF A 5 MN HYDRAULIC FORCE STANDARD MACHINE
Carlos Alberto Fabricio Jr. 1, Antonio Carlos M. Garcia 2, Manuel Antonio P. Castanho 3, Crhistian Raffaelo Baldo 4
1
Instituto de Pesquisas Tecnológicas (IPT), São Paulo/SP, Brasil, carlosjr@ipt.br
Instituto de Pesquisas Tecnológicas (IPT), São Paulo/SP, Brasil, garcia@ipt.br
3
Instituto de Pesquisas Tecnológicas (IPT), São Paulo/SP, Brasil, manet@ipt.br
4
Instituto de Pesquisas Tecnológicas (IPT), São Paulo/SP, Brasil, crbaldo@ipt.br
2
Abstract: Accurate force measurements by universal testing
machines and force transducers (e.g., load cells) are required
in many business sectors: oil and gas, civil, railways, and so
forth. The exploitation of the pre-salt layer oil has imposed
technical challenges such as the integrity evaluation of chain
cables used to anchor petroleum floating units. Large forces
may be involved, which need to be conveniently evaluated.
Some technical details and performance assessment related
to the ongoing upgrading of a hydraulic standard machine
for large forces are outlined and discussed in this paper.
They include the machine reinstallation in a new laboratory,
improvement in the calibration setup and preliminary results
of the current infrastructure.
(d) The Port and Logistics Sector uses force transducers
for load testing of cranes and towboats, for calibrating
load cells attached to cranes.
(e) The Railways Sector demands metrological evaluation
of load cells for weighing wagons and for measuring
tensile strength.
Other business sectors demand force measurements to
verify testing machines utilized to evaluate the strength of
materials (e.g., plastics, metals, composites, textiles, woods,
ceramics, foams, adhesives) and to assure quality control in
production lines. Fig. 1 shows the force measuring range for
several examples from industry.
Key-words: large force metrology, traceability, calibration,
hydraulic force standard machine, build-up system.
1. INTRODUCTION
The General Conference on Weights and Measures
(CGPM) established in 1901 that force is derived from the
basic units of mass, length, and time. In 1960, the CGPM
adopted the newton as the unit of force in the International
System of Units (SI), where one newton is the force required
to accelerate a mass of one kilogram to one m/s2, expressed
in terms of SI base units as kg ∙ m ∙ s-2 [1].
Even though force is a derived unit, the realization and
dissemination of the unit is of primary concern in many
application cases. In fact, reliable force measurements using
force transducers, load cells and universal testing machines,
are required in many business sectors, such as:
(a) The Oil and Gas Sector makes use of force transducers
for weighing petroleum platform modules, for load
proofing of cranes and towboats, for calibrating load
cells attached to cranes, for calibrating testing devices
devoted to cable and belt investigation.
(b) The Construction Sector claims for the metrological
evaluation of hydraulic jacks and force measuring
instruments applied to load test of structures (bridges,
buildings, tunnels, anchoring) [2].
(c) The Aerospace Sector requires force measurement for
aircraft weighing, as well as mounting parts of the
turbine engines and landing gears; which are subjected
to significant large force magnitudes during operation.
Fig. 1. Typical force measurement magnitudes for different business
sectors - adapted from [3]
The exploitation of the pre-salt layer oil has faced many
technical challenges. One of them is related to the integrity
of chain cables, which are used to anchor floating units.
Since they are subjected to very large forces, their integrity
needs to be ensured in order to avoid hazard to the platform
in the event of a breakage. For this reason, the metrological
confirmation of platform anchoring elements for breakage
load plays an important role in the whole process.
Significant efforts have been undertaken by the Institute
for Technological Research of the State of São Paulo - IPT
to enhance their capabilities in large force metrology. The
target of IPT is to realize the force quantity with the relative
uncertainty figures illustrated in Fig. 2. For the range up to
0.3 MN, load cells are calibrated by direct comparison with
our deadweight machine of rated capacity 0.3 MN, which is
calibrated by the Brazilian NMI; for forces above 0.3 MN,
force transducers are calibrated using the build-up method.
This paper outlines some technical and metrological details
of the modernization of a hydraulic force standard machine
with nominal loading range to 5 MN, which will contribute
to the realization of force to 9 MN. The first experimental
findings related to the already-implemented improvements
are presented and discussed.
measurement uncertainty, it is a must to maintain the
measuring volume under controlled temperature.
(c) Introduction of closed-loop control system and data
acquisition automation: one of the major issues of the
current arrangement is the operational efficiency. To
make the measuring equipment more appropriate for
practical use, and reduce operator effects, automation
solutions are needed.
(d) Metrological and operational validation of the renewed
machine: the impact of the proposed solution is to be
confirmed through performance tests and estimation of
the measurement capability.
3. EXPECTED AND PRELIMINARY RESULTS
Fig. 2. Relative uncertainty for the corresponding measuring ranges
(DWM: deadweight machine; BUS: build-up system)
2. TOPICS OF CONCERN
The modernization of the 5 MN hydraulic force standard
machine shown in Fig. 3 in response to increasing demand
for large force realization is the main object of this paper.
The original machine structure consists of a fixed robust
base frame with a four-column movable loading frame (noncontrolled driver). The machine modernization plan includes
the following macro activities:
The machine upgrading project has been put into action
since the beginning of this year, after transferring the entire
machine to the new IPT laboratory for heavy structure tests.
In parallel to the specification of all new machine parts and
the basic project of the equipment enclosure, improvements
have been already executed.
Basically they involved replacing the original indication
pointer (mechanical and analogical) by a pressure transducer
with digital display, thus enabling measurement automation,
and enhancing the build-up technique as a whole. The buildup approach allows calibrating either load cells or hydraulic
force machines of higher capacity by using three or more
load cells of equal and lower nominal capacity arranged in
parallel to each other [4-6].
One could then define three measurement states, which
are explored in the remaining paragraphs. The results of the
force machine calibration to the entire load range with the
original structure as per ISO 7500-1 [7] are summarized in
Table 1 (testing machine of class 1). Five load cells of 1 MN
nominal range, all them calibrated in conformity with ISO
376 [8], were grouped in order to form a 5 MN build-up
system. The largest relative deviations were observed in the
lower-end of the range (accuracy to 1.0%). The same remark
could be applied to the relative uncertainties (uncertainty to
0.8%).
Table 1. Summary of the calibration results for the original machine
using five load cells of equal rated capacity in a build-up system
Fig. 3. Illustrative drawing of the hydraulic force standard machine
with 5 MN nominal loading capacity
(a) Reengineering of mechanical and hydraulic parts of the
measuring equipment: the original four-pillar structure
and the material used in the hydraulic machine ensure
an excellent mechanical stiffness and through changes
in the hydraulic system the measurement capability can
be significantly improved.
(b) Improvement of the environmental conditions in the
measuring envelope: temperature variation may affect
calibration results of force transducers. To diminish the
Machine
Indication
(tf)
50
60
80
100
150
200
250
300
350
400
450
500
Reference
Value
(tf)
49.6
59.4
79.3
99.0
148.8
198.4
248.3
298.2
347.8
397.8
448.2
498.7
Acc.
(%)
0.9
1.0
0.9
1.0
0.8
0.8
0.7
0.6
0.6
0.5
0.4
0.3
Error
Repeat.
(%)
0.4
0.6
0.5
0.5
0.5
0.5
0.4
0.4
0.1
0.0
0.3
0.1
Uncertainty
(tf)
0.3
0.3
0.4
0.7
0.8
1.5
1.6
1.9
0.7
0.7
1.4
0.7
(%)
0.6
0.5
0.5
0.7
0.5
0.8
0.6
0.6
0.2
0.2
0.3
0.1
Considering only the measurement principle change, i.e.,
force values deriving from readings of a pressure transducer,
calibration results have complied with testing machine class
0.5. By attaching a pressure transducer to the machine, one
could also make profit of automatic data acquisition through
a digital display communicating in real time to a computer.
Table 2 points out the calibration results for the second case,
in which the relative errors are significantly lower than those
checked for the previous investigation. One could extend the
same remark to the relative uncertainties.
Table 2. Summary of the calibration results for the changed machine
using five load cells of equal nominal capacity in a build-up system
Machine
Indication
(kN)
300
400
500
600
800
1000
1500
2000
2500
3000
3500
4000
4500
5000
Reference
Value
(kN)
300.0
399.9
500.1
599.9
802.1
1003.0
1503.4
2002.9
2506.4
3008.9
3500.5
4001.0
4512.6
5007.2
Acc.
(%)
0.0
0.0
0.0
0.0
-0.3
-0.3
-0.2
-0.1
-0.3
-0.3
0.0
0.0
-0.3
-0.1
Error
Repeat.
(%)
0.1
0.3
0.3
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.4
Uncertainty
(kN)
0.6
1.3
1.4
1.2
2.9
3.4
4.1
5.2
6.9
9.7
11.4
15.9
14.3
20.4
(%)
0.2
0.3
0.3
0.2
0.4
0.3
0.3
0.3
0.3
0.3
0.3
0.4
0.3
0.4
The build-up system for machine calibration up to 5 MN
has been recently enhanced by replacing the five load cells
of similar capacity in parallel by three load cells with rated
capacity of 3 MN (in fact, the new build-up arrangement has
a maximum nominal capacity of 9 MN). That has involved
developing new parts for the build-up system, in particular
pendulums and mounting bases.
In the third experimental case, the machine calibration
was undertaken with a single force transducer for the range
up to 3 MN, and with the build-up system for the upper-end
measuring range. One could observe a valuable reduction of
the relative error and relative uncertainty in comparison with
the former situation.
Table 3. Summary of the calibration results for the changed machine
using three load cells of equal nominal capacity in a build-up system
Machine
Indication
(kN)
50
125
250
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Reference
Value
(kN)
50.0
125.0
249.6
499.7
1001.7
1503.4
2005.1
2570.0
3010.0
3511.8
4014.4
4518.8
5022.1
Acc.
(%)
0.0
0.0
0.2
0.1
-0.2
-0.2
-0.3
-0.3
-0.3
-0.3
-0.4
-0.4
-0.4
Error
Repeat.
(%)
0.3
0.3
0.3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
Uncertainty
(kN)
0.2
0.4
0.8
1.1
2.1
3.5
5.1
5.6
5.8
8.0
9.1
12.6
10.4
(%)
0.4
0.3
0.3
0.2
0.2
0.2
0.3
0.2
0.2
0.2
0.2
0.3
0.2
For results showed in Table 1 and Table 2 the main setup
difference was the use of a pressure transducer instead of the
original mechanical pointer. That resulted in improving the
uncertainty attributed to the machine display resolution by
25 times and hence enhancing machine accuracy.
For Table 3 the values are related to the calibration using
a three-load-cell-based build-up system (illustrated in Fig. 4)
in place of the five-load-cell-based build-up system used in
the first two tests.
For all cases the following uncertainty contributors were
regarded: (a) reference load cell(s) uncertainty stated in the
calibration certificate; (b) calibration process repeatability;
(c) reference standard display resolution; (d) machine-undercalibration display resolution; (e) thermal effect; (f) load cell
stability over time; (g) machine hysteresis.
Fig. 4. Complete three-load-cell-based build-up system during force
machine calibration
The graphs shown in Fig. 5 make clear the differences
between machine errors found with the original machine
(blue lines with diamond-type marker) and the machine
outfitted with a pressure transducer (red lines with squaretype marker). In addition to the uncertainty bar shortening
caused by measuring variation reduction, the measurement
bias was systematically smaller for the entire range.
Comparing the results of the five-load-cell-based buildup system with the three-load-cell-based build-up system,
all values are compatible (uncertainty bar overlapping). The
main advantage of the newer build-up arrangement (green
lines with triangle-type marker in Fig. 5) over the previous
setup is the reduction of the calibration uncertainty. The
newer build-up setup also has produced more consistent
outcomes, as relative uncertainties keep reasonably constant
over the entire measuring range.
4. CONCLUDING REMARKS
Through the machine modernization one has expected to
reduce calibration process lead time by 50% and increase
confidence in the calibration results according to ISO 75001 and ISO 376. This could be attained by including a closedloop control system and by improving the data collection
process. In fact, thanks to the pressure transducer assembled
directly to main piston-cylinder, data acquisition process has
been already improved. Operator effect on the measurement
result has been minimized and throughput increased. Due to
increasing demand for calibration services over large
magnitude forces, improving productivity is a factor that
needs to be taken into account.
Fig. 6. Sketch of the temperature-controlled room for the hydraulic
force standard machine in order to improve calibration confidence
More reliable calibration results also require redesigning
and changing some parts and confining the test volume. Fig.
6 illustrates the sketch of the temperature-controlled room,
where part of the machine column is not conditioned since it
has no practical use in the calibration of force transducers.
One has estimated to improve the best measuring capability
by 20% for direct measurements. The results obtained thus
far after replacing mechanical parts, measuring standards,
and redesigning the build-up system are very convincing.
From the application point of view, that means to be very
promising the intent of extending our calibration service
portfolio to some more critical cases.
REFERENCES
Fig. 5. Machine calibration results for three different scenarios: (a)
original machine (blue lines, diamond-type marker); (b) improved
machine (red lines, square-type marker); (c) new build-up system
setup (green lines, triangle-type marker)
The insertion of closed-loop control system would allow
speeding up the calibration procedure of load cells of higher
loading capacity (limited to rated capacity of the hydraulic
force machine used as comparator) with better uncertainty
using lower capacity force transducers. Predefined working
ranges could be also defined and a look-up table created to
automatically correct systematic errors on each individual
testing point.
In fact, the expected outcomes of the hydraulic force
standard machine modernization are most driven by market
needs. The petroleum exploitation in the pre-salt area is just
one of the major business sectors demanding large force
measurement solutions. To enhance the measurement and
operation capability is highly necessary in a competitive
market. More reliable products and processes in terms of
performance, quality and safety would necessarily require
more reliable force measurements.
[1] Z. J. Jabbour, S. L. Yaniv, “The kilogram and measurements of
mass and force”, Journal of Research of the NIST, Vol. 106,
No. 1, Jan-Feb 2001.
[2] B. Katz, P. Kornhauser, S. Bitas, “Calibration of hydraulic
force machines – requirements, concepts, problems, solutions”,
XIX IMEKO World Congress, Lisbon, Portugal, September,
2009.
[3] A. Schäfer, “Examples and proposed solutions regarding the
growing importance of calibration of high nominal forces”,
IMEKO 2010 TC3, TC5 and TC22 Conferences, Pattaya,
Chonburi, Thailand, November, 2010.
[4] G. Schulder, U. Kolwinski, D. Schwind, “A new design of a 5.4
MN build-up system”, XVIII IMEKO World Congress, Rio de
Janeiro, Brazil, September, 2006.
[5] C. Ferrero, C. Marinari, R. Kumme, W. Herte, “PTB and
INRIM high force intercomparison up to 9 MN”, XVIII
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2006.
[6] A. H. McIlraith, M. T. Clarkson, “A novel hydraulic force
standard machine”, Measurement Science and Technology,
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[7] ISO 7500-1, “Metallic materials - Verification of static
uniaxial testing machines - Part 1: Tension/compression
testing machines - Verification and calibration of the forcemeasuring system”, 2004.
[8] ISO 376, “Metallic materials - Calibration of force-proving
instruments used for the verification of uniaxial testing
machines”, 2004.